The present invention pertains generally to fluid pumps. More particularly, the present invention pertains to electro-osmotic pumps that are useful for transporting aqueous solutions in micro-fluidics. The present invention is particularly, but not exclusively, useful as a device and method for improving the pumping capacity of electro-osmotic pumps.
It is well known that a liquid can be moved through a small diameter tube under the influence of an applied electric field by a phenomenon that is commonly known as the electro-osmotic (EO) effect. Specifically, the EO effect arises from the fact that when an aqueous solution comes into contact with certain active materials (either acidic or caustic), the solution becomes, charged. If an acidic active material is used, such as silica, the solution becomes positively charged. On the other hand, if a caustic material is used, the solution becomes negatively charged. In either case, the application of an electric field on the charged solution will generate forces on the solution that cause it to move.
It happens with the EO effect that only a very thin layer of the solution that is in direct contact with the active material will become charged. Typically, this layer of charged solution will have a very shallow depth that is approximately equal to the Debye length (e.g. 10 nm). The consequence of this is that only a relatively small volume of the solution can be charged by the EO effect. Nevertheless, despite the small volume of charged solution, in order to be effective in moving an aqueous solution through a tube, the forces that are generated on the charged solution by an applied electric field must somehow overcome the pressure head in the tube.
For micro-fluidics applications it is well known that the EO effect can be usefully employed, but with some significant limitations. Most noticeably, these limitations involve the size of the tubes that can be used, and the magnitude of the electric field that can be used to drive the charged aqueous solution through the tube. Specifically, insofar as the electric field is concerned, high current densities for generating this electric field are undesirable for at least two reasons. First, high current densities can cause excessive ohmic heating of the solution in the tube. Second, the high current densities at the electrodes that generate the electric field may evolve gases in the tube due to the electrolysis of water. This, in turn, will disrupt the electric field. Insofar as the size of the tubes is concerned, the pressure head in the tube that resists the movement of liquid through the tube is of paramount importance. Heretofore, for the EO effect to be useful in overcoming pressure head, small diameter tubes have been required (typically the radius must be less than 10-20 microns). With this in mind, a mathematical analysis of the EO effect, and its interaction with the resistive pressure head in the tube, is instructive.
For an example of conventional flow in a tube due to the EO effect, in resistance to a pressure head, consider a tube which is made of an EO active material, such as silica, and which has a lumen of radius xe2x80x9caxe2x80x9d.
The bulk flow velocity of the EO flow that is driven by an electric field, within a thin layer near the wall of the tube, is given by
u=xcexxcexa3V/2xcex7L
where xcex is the layer thickness (typically 10 nm), xcexa3 is the wall surface charge density (typically 10xe2x88x922 Coulomb/m2), V is the voltage, xcex7is the absolute viscosity absolute viscosity of the fluid and L is the length of the tube. The velocity can be written in terms of zeta potential xcex6 defined as
xcex6=xcexxcexa3/∈
where ∈ is the dielectric constant of the fluid.
The Poiseille flow which is driven by the pressure head, and which resists the EO flow described above, has a parabolic velocity profile given by
v=uxe2x88x92[pa2/4Lxcex7][Ixe2x88x92r2/a2]
where p is the pressure head, and where a value for a  greater than  greater than xcex is assumed. Under these conditions, total flow discharged role in the tube is given by
xe2x80x83xcex93=∫0a2xcfx80v r dr=xcfx80a2{uxe2x88x92pa2/[8Lxcex7]}.
The condition that the EO drive overcomes the pressure head is then given by
a2 less than 4xcexxcexa3V/p.
From the above expression it will be appreciated that when a large pressure head is desirable, the radius of the tube xe2x80x9caxe2x80x9d must be quite small. The consequence is a very small throughput. The optimal radius with other parameters fixed is given by
a2=2xcexxcexa3V/p
and the total flow becomes
xcex93=xcfx80a2u/2.
From the above expression, it is to be appreciated that the electro-osmotic (EO) effect is a surface effect. As such, the EO effect is significantly dependent on the amount of surface area of the active material that is exposed to the aqueous solution.
In light of the above, it is an object of the present invention to provide a tubular shaped electro-osmotic pump for pumping an aqueous solution which effectively increases the amount of active material surface area that is exposed to the solution per length of tubing used. Another object of the present invention is to provide a tubular shaped electro-osmotic pump which can effectively employ lumens of increased cross sectional areas. Yet another object of the present invention is to provide an electro-osmotic pump which has increased efficiency with little or no increase in voltage requirements in order to avoid ohmic heating of the pump and the unwanted evolution of gas due to electrolysis. Still another object of the present invention is to provide an electro-osmotic pump that can be variously used as a switch or a valve, as well as a pump. Another object of the present invention is to provide an electro-osmotic pump that can effectively incorporate a trapped air isolator which will prevent clogging of the active element of the pump, and maintain low electrical conductivity. Also, it is an object of the present invention to provide an electro-osmotic pump that is relatively simple to manufacture, is easy to use, and is comparatively cost effective.
The electro-osmotic pump of the present invention provides structure which significantly increases the interface surface area between an active element (e.g. silica fibers) and an aqueous solution in which the active element is submerged. Consequently, more of the aqueous solution can be charged by the active element, and a lower electric field charge is effective for generating a pumping force on the solution.
In accordance with the present invention, a container is provided for holding an active element in an aqueous solution. Preferably, the container is tube-shaped and has a lumen which defines an axis that extends from one end of the tube to the other. In the preferred embodiment of the present invention, the active element will include a plurality of fibers that are spun together into a thread. This thread is then positioned inside the lumen of the tube-shaped container to create a pump tube. Importantly, the thread will extend between the ends of the pump tube with the fibers of the thread aligned substantially parallel to the axis of the pump tube. The lumen of the pump tube is then filled with an aqueous solution that will interact with the thread to charge the aqueous solution. As envisioned for the present invention, the cross sectional area of the pump tube lumen, taken in a plane perpendicular to the axis of the pump tube, will have an area equal to xe2x80x9cAxe2x80x9d, while the collective cross sectional areas of the fibers in the thread in this plane will be equal to approximately one half of xe2x80x9cAxe2x80x9d (i.e. A/2).
In order to create an electric field in the lumen of the pump tube, electrodes are positioned at each end of the pump tube. Preferably, one of these electrodes will have a zero potential while the other electrode has either a negative or a positive potential and the resultant electric field will be oriented substantially parallel to the axis of the pump tube. Accordingly, whenever an electric field is applied to the pump tube, a force will be created on the charged aqueous solution that will move the aqueous solution through the pump tube.
In combination, an extension tube can be connected in fluid communication to one end of the pump tube. Importantly, depending on whether the extension tube is connected to a voltage potential V or zero potential (ground) at the end of the pump tube, the extension tube will respectively return from a zero potential (ground) to the voltage potential V or vice versa. Together, a pump tube and the extension tube will then define a pumping section for the electro-osmotic pump of the present invention. Further, in order to increase the pumping force of the electro-osmotic pump, a plurality of these pumping sections can be serially joined together with an alternation between pump tubes and extension tubes. Importantly, because voltages can be applied in parallel to the serially connected pumping sections, there is no requirement for using higher voltages.
An important option for the present invention involves the extension tube. For one embodiment, the extension tube can be filled with the aqueous solution. This, however, is not a requirement. Specifically, for situations wherein it may be desirable to pump a fluid other than the aqueous solution, the extension tube may be at least partially filled with an air bubble. The air bubble will then isolate the aqueous solution and thread in the pump tube from whatever different fluid is in the extension tube and is being pumped by a pumping section. Other options for the present invention involve various orientations for the pump and extension tubes, as well as changes in their respective cross sectional areas. As envisioned for the present invention, these various orientations and changes can allow the electro-osmotic pump of the present invention to be used as a valve or a switch in addition to its more conventional use as a pump.